1. Field of the Invention
The present invention relates to a broadband wireless communication system based on an OFDM (Orthogonal Frequency Division Multiplexing) scheme, and more particularly to an apparatus and method for generating a pilot pattern to distinguish between BSs (Base Station).
2. Description of the Related Art
A transmitter, for example a base station (BS), for use in a conventional OFDM communication system transmits pilot sub-carrier signals (pilot channel signals) to a receiver, for example a mobile station (MS). The BS transmits the data sub-carrier signals (data channel signals), and at the same time transmits the pilot channel signals. The reason the pilot channel signals are transmitted is to perform synchronization acquisition, channel estimation, and BS distinguishment.
The OFDM scheme available for high-speed data transmission in wired/wireless channels serves as an MCM (Multi Carrier Modulation) scheme, which transmits data using a multi-carrier, converts serially-received symbol streams into parallel symbol streams, modulates each symbol stream into a plurality of sub-carriers (i.e. a plurality of sub-channels) orthogonal to each other, and finally transmits the plurality of sub-carriers.
Such an MCM system was first applied to a high-frequency radio system for use in the military in the late 1950s, and the OFDM scheme for overlapping between a plurality of orthogonal sub-carriers was first studied in the late 1970s. This OFDM scheme must implement orthogonal modulation between the multi-carriers, resulting in limited system application.
The modulation/demodulation based on the OFDM scheme was developed by Weinstein in 1971, and is processed using a DFT (Discrete Fourier Transform). Many developers have conducted intensive research into the OFDM scheme. Usage of a guard interval and a method for inserting a cyclic prefix guard interval are well known to those skilled in the art. The use of a guard interval greatly reduces the negative influence on a system affected by a multi-path and a delay spread.
Therefore, the OFDM scheme is widely applied to the digital transmission technology, for example DAB (Digital Audio Broadcasting), digital TV, WLAN (Wireless Local Area Network), and a WATM (Wireless Asynchronous Transfer Mode). Although the use of the OFDM scheme has been limited due to its hardware complexity, the OFDM scheme can be implemented with digital signal processing technology such as an FFT (Fast Fourier Transform) and IFFT (Inverse Fast Fourier Transform).
The OFDM scheme is similar to a conventional FDM (Frequency Division Multiplexing) scheme, but it can obtain an optimum transmission efficiency during a high-speed data transmission because it transmits a plurality of sub-carriers that are orthogonal to each other. Further, the OFDM scheme has a superior frequency use efficiency and is very resistant to a multi-path fading, resulting in an optimum transmission efficiency during a high-speed data transmission.
Because the OFDM scheme uses an overlapped frequency spectrum, it can effectively use a frequency, is very resistant to a frequency selective fading and a multi-path fading, reduces intersymbol interference (ISI) using a guard interval, and allows for the use of an equalizer composed of simple hardware. Also, the OFDM scheme is very resistant to impulse noise, such that it is widely adapted to communication system architecture.
The pilot channel signals act as a training sequence, perform channel estimation between a transmitter and a receiver, and allow an MS to determine a BS to which the MS belongs using the pilot channel signals. The position where the pilot channel signals are transmitted is determined between the transmitter and the receiver. The pilot channel signals act as reference signals.
A pattern generated by the pilot channel signals transmitted from the BS is called a pilot pattern. The pilot patterns for use in the conventional OFDM system are distinguished by slopes of the pilot channel signals and start points at which the pilot channel signals begin their transmissions. The OFDM communication system must be designed for individual BSs to have different pilot patterns such that the BSs for the OFDM communication system can be distinguished from each other.
The pilot patterns are generated by taking into consideration a coherence bandwidth and a coherence time. The coherence bandwidth is indicative of a maximum bandwidth on the assumption that the same channel is used in a frequency domain (i.e. a channel remains unchanged in the frequency domain). The coherence time is indicative of a maximum time on the assumption that the same channel is used in a time domain (i.e. a channel remains unchanged in the time domain).
It can be assumed that the same channel is used for the coherence bandwidth and the coherence time, such that there are no problems associated with synchronization acquisition, channel estimation, and BS distinguishment even though the coherence bandwidth and the coherence time transmit only one pilot channel signal. Further, the transmission of the data channel signals can be maximized, resulting in improved overall system performance.
A minimum frequency interval for the transmission of pilot channel signals is indicative of a coherence bandwidth. A minimum time interval (i.e. a minimum OFDM symbol time interval) for transmitting the pilot channel signals is indicative of a coherence time.
Although the number of BSs contained in the OFDM communication system is variable with the size of the OFDM communication system, the larger the size of the OFDM communication system, the greater the number of the BSs. In order to distinguish the BSs from each other, the number of pilot patterns having different slopes and different start points must be equal to the number of the BSs.
An example of a variety of pilot channel patterns will be described with reference to FIG. 1.
FIG. 1 depicts all of the slopes that can be generated in the form of a pilot channel pattern in the conventional OFDM communication system.
Referring to FIG. 1, the slopes that can be generated in the form of a pilot channel pattern and the number of the slopes (i.e. the slopes in response to pilot channel signal transmission, and the number of the slopes) are limited by the coherence bandwidth 101 and the coherence time 102.
Assuming that the coherence bandwidth 101 is determined to be ‘6’, the coherence time 102 is determined to be ‘1’, and the pilot pattern slope is an integer, a slope S of a pilot pattern that can be generated is determined to be S=0, S=1, S=2, S=3, S=4, and S=5, such that the number of the slopes S is equal to ‘6’. In more detail, the slope S of the pilot pattern that can be generated is determined to be one of the integers 0˜5.
In this case, if the number of pilot pattern slopes is ‘6’, this means that the number of BSs distinguishable by the pilot pattern in the OFDM communication system satisfying the aforementioned assumption is ‘6’.
A pilot sub-carrier having a pilot pattern slope S of 6 (i.e. S=6) will hereinafter be described. Indeed, a first case in which the pilot pattern slope S is 0 and a second case in which the pilot pattern slope S is 6 are not distinguished from each other, such that only one of the first case and the second case is available. The pilot sub-carrier having the pilot pattern slope S of 6 can also be denoted by S=0, in which S is indicative of another pilot pattern slope S spaced apart from the pilot sub-carrier by the coherence bandwidth 101, such that the first case of S=0 and the second case of S=6 are not distinguishable from each other.
The hashed circle of FIG. 1 is a pilot sub-channel signal spaced apart from the pilot sub-carrier by the coherence bandwidth 101. If the slope S of the pilot sub-carrier denoted by the white circle is denoted by S=6, then the slope S of the pilot sub-carrier denoted by the hashed circle can also be determined to be ‘0’. The slope of the pilot sub-carrier is limited to the coherence bandwidth 101.
An OFDMA-CDM (Orthogonal Frequency Division Multiple Access-Code Division Multiplexing) system will hereinafter be described.
FIG. 2 depicts a method for dividing time-frequency resources in the OFDMA-CDM system. Referring to FIG. 2, a unit square is composed of predetermined sub-carriers (e.g., 8 sub-carriers), and is defined as a TFC (Time-Frequency Cell) 201 having the same duration as a single OFDM symbol interval. A Frame Cell (FC) 203 is defined as a time-frequency domain, which includes a bandwidth equal to an integer multiple (e.g., 16 times) of the TFC 201 and a duration equal to an integer multiple (e.g., 8 times) of the TFC 201.
FCs of FIG. 2 are classified into FCs for packet data transmission and other FCs for the transmission of control information associated with the sub-channels over which the packet data is transmitted. Two sub-channels with different hopping patterns of a predetermined frequency interval are shown in the packet data transmission FC. In more detail, the sub-channel A and the sub-channel B are shown in FIG. 2.
In more detail, the OFDMA-CDM scheme shown in FIG. 2 adapts the characteristics of the OFDM scheme and the other characteristic of the CDMA scheme to maximize a performance gain. The total bandwidth in the OFDMA-CDM scheme is divided into a plurality of sub-carrier domains (i.e. a plurality of sub-frequency domains).
The frequency domain ΔfTFC, which has the same duration ΔtTFC as the OFDM symbol interval and is composed of predetermined sub-frequency domains, is defined as a TFC 201. The TFC 201 is composed of predetermined sub-frequency domains.
The number of sub-frequency domains that comprises the TFC 201 varies with system conditions. The frequency domain occupied by the TFC 201 is defined as a TFC frequency domain, and the time domain occupied by the TFC 201 is defined as a TFC time interval. As a result, the unit squares 201 shown in FIG. 2 depict the TFCs.
The data processing step based on the CDMA scheme includes a step for spreading data by a channelization code assigned to every sub-carrier and/or a step for scrambling the spread data using a predetermined scrambling code.
A single FC 203 is comprised of the TFCs. The FC 203 includes a bandwidth ΔfFC corresponding to a predetermined multiple, instead of including ΔfTFC indicative of the bandwidth of the TFC 201, and includes a duration ΔtFC corresponding to a predetermined multiple, instead of including ΔtTFC indicative of the duration of the TFC. For example, the FC 203 includes a bandwidth corresponding to 16 times of the TFC 201's bandwidth ΔfTFC as denoted by ΔfFC=16 ΔfTFC, and includes a duration corresponding to 8 times of the TFC's duration ΔtTFC as denoted by ΔtFC=8ΔtTFC. In this case, the frequency domain occupied by the FC is defined as an FC frequency domain, and the time domain occupied by the FC is defined as an FC time domain.
The first FC to the (M−1)-th FC from among the M FCs are adapted to transmit packet data, an M-th FC may be adapted to transmit control information. Needless to say, the number of FCs used for the packet data transmission and the number of FCs used for the control information transmission are variably determined according to the system conditions. The greater the number of the FCs used for the control information transmission, the lesser the number of the FCs used for the packet data transmission, which results in a problem of data transfer rate deterioration. The number of the FCs used for the packet data transmission and the number of the FCs used for the control information transmission are determined taking into consideration this problem. For the convenience of description, the FC used for the packet data transmission is called a data FC (Frame Cell), and the other FC used for the control information transmission is called a control FC (Frame Cell).
Two different sub-channels (i.e. the sub-channel A and the sub-channel B) in a single FC are shown in FIG. 2. In this case, the sub-channel is indicative of a transmission channel in which the predetermined TFCs are frequency-hopping processed with time according to a predetermined frequency hopping pattern. It should be noted that the number of TFCs contained in the sub-channel and the frequency hopping pattern are variably determined depending on the system conditions. FIG. 2 depicts an exemplary case in which a single sub-channel is composed of 8 TFCs.
In the case of adapting a pilot transmission method of the conventional OFDM system to the OFDMA-CDM system, pilot spreading is not performed whereas data spreading is performed, such that a method for multiplexing the data signals and pilot signals becomes troublesome. A variety of intervals (i.e. various intervals from a narrow interval to a wide interval) from one pilot signal to its neighboring pilot signal must be used in the frequency domain to increase the number of pilot signal patterns in such a way that the BSs can be distinguished from each other. In this case, the interval between the pilots signals may be greater than the coherence bandwidth. If BS distinguishment and channel estimation are performed using the pilot signals, channel estimation performance may be deteriorated.